As it is generally known, Operations Administration and Management (OAM) is a standard term referring to tools used to monitor and troubleshoot a network. OAM for switched Ethernet networks is being standardized in IEEE 802.1ag under the name “Connectivity Fault Management” (CFM), and in ITU-T SG13 under the name “OAM Functions and Mechanisms for Ethernet based networks”. Typically, Ethernet CFM is defined specifically for fault management for Ethernet and it does not work for other types of networks such as, Virtual private LAN services (VPLS) over Multi-protocol Label Switching (MPLS), which requires a specific MPLS OAM. Service provider networks are increasingly using/building mixed networks including switched Ethernet and MPLS.
FIG. 1 is a block diagram illustrating a typical network configuration having a multi-tiered network. In this example, switched Ethernet 101 is coupled to a VPLS/MPLS network 102 via a provider edge (PE) router 104, where a PE is also referred to as a label edge router (LER). Similarly, switched Ethernet 103 is coupled to the VPLS/MPLS 102 via 105. Within the VPLS/MPLS network 102, one or more core routers (also referred to as P routers) 106-107 are used to route MPLS packets between PEs 104-105.
When an Ethernet CFM operation is initiated, for example, Ethernet node 108 sends a link-trace message (LTM) as described in 802.1ag CFM to PE 104. The original LTM 110 is destined to a destination Ethernet node 109 (e.g., destination media access control or MAC address of node 109). In response, PE 104 responds with a link-trace reply (LTR) message having a source MAC address of PE 104 and forwards the LTM message to PE 105 via pseudo-wire. In addition, PE 105 also responds with an LTR after receiving the LTM from PE 104 having a source MAC of PE 105 and forwards the original LTM to node 109. Assuming there is no fault in the network, in response to the original LTM, node 109 responds with an LTR having a source MAC address of node 109, which is routed back to the originator node 108. Thus, all LTRs are received by node 108 from PEs 104-105 and node 109.
FIGS. 2A-2B are block diagrams illustrating certain fault scenarios in a network configuration of FIG. 1. Referring to FIG. 2A, it is assumed that there is a link fault in a path between PE 105 and node 109. In this scenario, after sending an LTM, although the originating node 108 receives LTRs from PE 104 and PE 105, node 108 never receives an LTR from node 109. In this example, node 108 may be able to determine that there is an error in a link between PE 105 and node 109 or the problem is with node 109.
However, if there some errors occur within the VPLS/MPLS network 102 as shown in FIG. 2B, the situation may be more complicated in which node 108 may not be able to determine the location of the errors. Referring to FIG. 2B, in this example, it is assumed that there is an error occurred in a link between core routers 106-107. When PE 104 receives an LTM from node 108, PE 104 forwards the LTM to the corresponding destination, node 109. Since the link between core routers 106 and 107 is broken, PE 105 and node 109 never receive the forwarded LTM. As a result, corresponding LTRs from PE 105 and node 109 are not received by PE 104 by the originating node 108. Thus, node 108 cannot determine whether there is any link error among the links between PE 104 and PE 105, or alternatively the problem could be with PE 105.